Smart laser with fast deformable grating

This is a continuation-in-part application of Ser. No. 09/390,579; filed Sep. 3, 1999 now U.S. Pat. No. 6,212,217. This invention relates to lasers and in particular to lasers with feedback control of beam quality.

In many laser applications precise control of beam output is desired. One such application for such lasers is the light source for integrated circuit lithography. Currently the KrF excimer laser is the choice light source for state of the art integrated circuit lithography devices. Specifications for the light source are becoming tighter as efforts are made to increase production and produce finer integrated circuit patterns.

Typical specifications for a 248 nm KrF laser call for bandwidths of about 0.6 pm full width half maximum, wavelength stability within 0.1 pm of the specified wavelength and energy dose stability of about ±0.5 percent. In addition, control of beam cross section intensity values are important.

FIG. 1 shows some of the features of a prior art KrF excimer laser system used for IC lithography. The system includes a laser frame structure 5 within which is mounted a laser chamber 3 containing two elongated electrodes (not shown) between which is a gain medium, a line narrowing module (referred to as a “line narrowing package” or LNP) 7 shown disproportionately large and an output coupler 4 . The LNP portion of FIG. 1 represents a top view of the LNP. The beam cross section is generally rectangular, typically about 3.5 mm wide and about 15 mm high. In prior art devices each of the line narrowing module 7 and the output coupler module 4 comprise frames which are fixedly mounted to laser frame structure 5 . Optical components within the frames of the output coupler module and the line narrowing module are adjusted manually to define the laser resonant cavity. The chamber is adjustably mounted within the laser frame so that it can be finely positioned manually within the defined resonant cavity from time to time in the direction of the beam width as shown by arrows 3 A on FIG. 1 . These adjustments permit a laser technician to align the resonance cavity with the gain medium in order to achieve optimum beam output parameters. In this prior art for example, a three prism beam expander 18 is comprised of prisms 8 , 10 and 12 mounted on prism plate 13 . In the prior art device, prism plate 13 can be manually adjusted in the direction of arrows 13 A as an alignment technique. The prior art device also includes a manual adjustment of the curvature of the surface of grating 16 into an increasingly or decreasingly concave shape by expanding or contracting bending mechanism 20 to place larger or smaller compressive forces on legs 17 A and 17 B. The adjustment is done primarily to control bandwidth of the output beam. Another prior art technique for forcing a concave shape on the grating surface is described in U.S. Pat. No. 5,095,492.

Typical prior art lithography excimer lasers now in use incorporate two automatic feedback controls to regulate pulse energy and nominal wavelength. Pulse energy is controlled in a feedback system by measuring the output pulse energy with a beam output monitor 22 as shown in FIG. 1 and then using these measurements with a computer controller 24 to control the high voltage applied between the electrodes in order to regulate pulse energy within desired limits. The beam output monitor 22 (also called a wavemeter) also measures the nominal or center wavelength and bandwidth of the pulsed output beam. Computer controller 24 adjusts the pivot position of tuning mirror 14 using stepper motor 15 in order to control the nominal wavelength of the beam to within desired limits.

What is needed are improvements which will provide easier, faster and more precise control of laser beam output parameters.

The present invention provides a smart laser having automatic computer control of pulse energy, wavelength and bandwidth using feedback signals from a wavemeter. Pulse energy is controlled by controlling discharge voltage, wavelength by controlling the position of an R MAX mirror and bandwidth is controller by adjusting the curvature of a grating to shapes more complicated than simple convex or simple concave. A preferred embodiment provides seven piezoelectric driven pressure-tension locations on the back side of the grating at 5 horizontal locations to produce shapes such as S shapes, W shapes and twisted shapes. Preferred embodiments include automatic feedback control of horizontal and vertical beam profile by automatic adjustment of a prism plate on which beam expander prisms are located and automatic adjustment of the R MAX tilt. Other preferred embodiments include automatic adjustment of the horizontal position of the laser chamber within the resonance cavity.

FIG. 1 shows a prior narrow band art laser configuration.

FIG. 2 shows a preferred embodiment of the present invention.

FIGS. 3A, B and C show the operational principals of a grating bending mechanism.

FIG. 4 shows a prospective view of the bending mechanism.

FIG. 5 shows some of the features of the FIG. 2 preferred embodiment.

FIGS. 6A, B, C and D show other features of the FIG. 2 embodiment.

FIGS. 7A and B show bottom prospective views of the line narrowing module of the FIG. 2 embodiment.

FIG. 8 shows an optical setup for measuring selected beam parameters.

FIGS. 9, 9 A, 9 B, 9 C, 9 D, 9 E and 9 F are drawings of a deformable grating.

FIGS. 10A, B, C and D show deformed gratings.

FIGS. 11A and B show a piezoelectric drive for a tuning mirror.

Preferred embodiments of the present invention can be described by reference to the drawings.

Smart Laser

A combination block diagram schematic drawing of a smart laser system is shown in FIG. 2 . This drawing shows important improvements over the prior art which automate the alignment of the laser chamber and components in order to provide greatly improved instant control of the important laser beam parameters. The new laser frame 5 A has added on to it a chamber position stepper motor to automatically adjust the horizontal position of the chamber in the direction 3 A. The LNP 7 A includes a prism plate stepper motor 32 , an R-max tilt stepper motor 34 and a grating curvature motor 30 . All of these stepper motors are controlled by computer controller 24 A.

Two-Way Automatic Control of Grating Surface Curvature

A grating curvature stepper motor 30 has been added to control the curvature of grating 16 . The system includes a new bending mechanism design 20 A which has the capacity to apply either a compressive force to spread apart legs 17 A and 17 B to create a concave curvature in the lined surface of grating 16 or a tension force to pull tog ether legs 17 A and 17 B to crate a convex curvature in the lined surface of grating 16 . Control of motor 30 is provided by computer controller 24 .

The basic elements and a functional description of the operation of the grating bending mechanism is shown in FIGS. 3A, 3 B and 3 C. FIG. 3A shows a grating assembly having a bidirectional control unit attached to it but with no bending force applied to the grating. Shown are grating 16 , left end plate 17 B, right end plate 17 A, compression spring housing 48 , left compression spring 50 , right compression spring 51 , adjustment shaft 44 and piston 49 which is fixedly pinned to adjustment shaft 44 . Adjustment shaft 44 comprises threaded length 44 A (¼-28 UNF-2B×1.38 long) which mates with threaded channel in right end plate 17 A. In the FIG. 3A condition, both springs are applying equal compressive force which offset each other or both springs may be unloaded. The curvature of the grating surface is adjusted by turning shaft 44 . By screwing shaft 44 in to housing 48 , left compression spring 50 is compressed against the left side of housing 48 and piston 49 as shown by the two arrows inside housing 48 in FIG. 3 B. The compression force pushes rod 44 to the right and housing 48 to the left which has the effect of pushing apart the two end plates 17 A and 17 B as shown by arrows 56 . This has the effect of bending the surface of grating 1 into a simple concave shape as shown by line 58 .

Conversely, by screwing shaft 44 in a direction to drive rod 44 out of housing 48 , right compression spring 51 is compressed against the right side of housing 48 and piston 49 as shown by the two arrows in side housing 48 in FIG. 3 C. The compressive force pulls rod 44 to the left and pulls housing 48 to the right which has the effect of pulling end plates 17 A and 17 B together as shown by arrows 57 . This has the effect of bending the surface of grating 1 into a simple convex shape as shown by line 59 .

In this system rod 44 has 28 threads per inch and the springs are rated at 52 pounds per inch. Operators are able with this design to make extremely fine adjustments to the curvature of the grating surface.

FIG. 4 is a perspective view of a grating assembly 16 A fabricated by Applicants and their co-workers. The assembly is comprised of grating 16 , two grating end plates 42 (bonded to grating 16 ) right bidirectional bandwidth control end plate 17 A, lock nut 56 , invar base plate 53 bonded to grating 16 , alignment rod 44 , socket 64 , two linear bearings 62 , compression spring housing 48 , right compression spring 51 , two thrust bearings 63 , piston 49 pinned to rod 44 , left compression spring 50 , travel limiting piston 57 pinned to rod 44 , radial ball bearing 54 , pivot shaft 55 and left bandwidth control end plate 17 B.

FIG. 5 is a cutaway drawing of LNP 7 A. It shows the two-way curvature-controlled grating assembly 16 A. Also shown is grating curvature control stepper motor 30 for controlling the curvature of the lined surface of grating 16 from concave to convex as explained above with reference to FIGS. 3A, 3 B and 3 C. FIG. 5 also shows prism plate adjustment motor 32 . Motor controls for R MAX mirror 14 are not shown in FIG. 5 .

Bottom views of line narrowing package 7 A are shown in FIG. 7A (from the front, i.e., looking from the laser toward the LNP) and in FIG. 7B (from the rear). Grating curvature stepper motor 30 is shown mounted on its mounting plate. Prism plate motor is shown at 32 and R MAX tilt motor is shown at 34 and R MAX stepper motor is shown at 15 . The beam entrance-exit port for the LNP is shown at 60 .

Prism Plate Position Control

Position control of prism plate 13 is depicted in cutaway drawing which also shows prism plate stepper motor 32 . Stepper motor 32 is also shown in FIGS. 7A and 7B mounted on its mounting plate. Control of motor 32 is provided by computer controller 24 .

Automatic R MAX Tilt Control

R MAX tilt control stepper motor is shown at 34 in FIGS. 7A and 7B and 6 A, C and D. The tilt of R MAX mirror 14 is provided by R MAX stepper motor 34 which is also controlled by computer controller 24 . The tilt of mirror 14 determines the vertical angle of light reflecting in the resonance cavity.

Wavelength Selection with Tuning Mirror

In this system, wavelength selection is provided with stepper motor 15 setting the pivotal horizontal position of tuning mirror 14 based on direction from computer controller 24 which utilizes feedback wavelength information from wavemeter 22 in accordance with prior art techniques as discussed in the background section of this specification.

Automatic Chamber Position Control

This system includes chamber position stepper motor 36 shown in FIG. 2 which automatically adjusts relative to frame 5 A (on which is mounted output coupler 4 and line narrowing package 7 A) the horizontal position of laser chamber 3 (and thus the horizontal position of the gain medium contained therein) in the direction perpendicular to the direction of beam 6 . Preferably two stepper motors 3 B are provided one positioned near the output (front) end of the chamber and the other near the LNP (back) end of the chamber. These two motors permit a chamber angular alignment of the axis of the discharge region with the optical axis of the laser system.

Controls

Computer controller is preferably programmed with control algorithms which control motors 36 , 32 , 34 , 30 in addition to 15 in order to maintain beam parameters within desired ranges based on feedback signals from wavemeter 22 . A simple approach is to hold all positions constant except one (for example chamber position stepper motor) and scan that item over a predetermined range to seek the position producing the optimum beam performance looking at parameters such as pulse energy output, pulse energy stability and bandwidth. The computer can be programmed to make these scans on operator instruction or to do the scans on a predetermined periodic basis. The computer could also be programmed to make one or more of these types of scans, seeking optimum position, if wavemeter detects a deterioration of any beam quality.

Also during burst mode operation of the laser (where, for example, the laser is operated to produce bursts of pulses such as 300 pulses at a rate of 1000 pulses per second followed by a downtime of 0.3 seconds) beam parameters are known to vary as a function of pulse number (i.e., time after the start of the bursts). In order to moderate or compensate for these variations, the computer controller could be programmed to adjust one or more of the stepper motors as a function of time after the start of the bursts.

Specific Optimization Techniques

In one preferred performance optimization technique a figure of merit M is defined in order to judge optimum laser performance. Adjustments are then made to maximize the value of the figure of merit. This value is computed using input from sensors which measure the beam in real time. These sensors typically provide values such as energy stability, laser efficiency (energy out for voltage in), bandwidth, beam width, beam symmetry, pointing stability, etc. In general the best figure of merit will combine the several parameters which are most important for determining success in the application, such as lithography exposure. For example if only laser efficiency as measured by pulse energy/charging voltage (E) was considered important to the figure of merit would be

M=pulse energy/charging voltage, or

M=E

If spacial symmetry (in the horizontal direction), SH, is to be judged in addition E, then S should be measured and given a weighting factor, W SH . Perfect symmetry would be zero. The new formula for figure of merit would then be:

Adjustments would then be made to maximize M. Similarly the figure of merit M could be made a function of other parameters such as vertical symmetry (VS), bandwidth (B), wavelength stability (WS) and dose stability (DS). In this case the formula for M would be:

M=E −( W SH )( SH )−( W SV )( SV )−( W B )( B )−( W WS )( WS )−( W DS )( DS )

Again, the computer is programmed to make adjustments to the stepper positions, measure E, SH, SV, B, WS and DS, apply weighting factors to achieve maximum figures of merit M.

Many techniques are well known for optimizing laser performance where several parameters of the type discussed above are considered. One preferred embodiment is the downhill simplex method which is documented in the book Numerical Recipes, The Art of scientific Computing by W. H. Press, et al., Cambridge University Press 1990 and referenced therein. In brief, a group of initial settings is chosen for the adjustments. There will be a number of configurations (a configuration is a set of values for the adjustments) which is one greater than the number of parameters being adjusted. For one iteration, the adjustments are set to each configuration and the figure of merit is measured. The configuration with the worst merit is then rejected and replaced with a new configuration which is closer to the best configuration. As the iterations proceed, the configurations become closer to one another until any one of them may be chosen as the optimum. In early work, Applicants have found that about 10 iterations suffice to locate the optimum. The downhill simplex method is a reliable technique, however, if very rapid convergence is needed other well known techniques could be utilized.

An Example of an Optimization Technique

The merit function M should be chosen so that adjusting the laser to obtain higher values of M will mean more effective operation, which could mean lower operational costs and/or higher optical performance in the application. Defining the “best” formula for M is a question that would require considerable research to determine, but it is not difficult to invoke a reasonably useful definition. For example, we may assume that energy efficiency (E/V) and bandwidth (E 95 ) are of primary interest, then we can represent a reasonable merit function as

M =( E/V )− W*E 95

Where M is the value of the merit function, E/V is the efficiency in mJ per volt, W is a weighting factor to adjust the relative sensitivity of the merit function to efficiency versus bandwidth, and E 95 is the laser bandwidth defined by the 95% integral width criterion.

Now assume that we have four adjustments to optimize on the laser, such as grating curvature (BCD), R-max tilt (RMAX), and chamber position (CH-front and CH-rear). This will constitute a multi-variable optimization process and in general we must expect interactions among the adjustments, so that for example the best setting for BCD will depend on the current adjustment for the RMAX. There are many standard algorithms for dealing with multi-variable optimization, such as the downhill simplex method or fuzzy logic approaches, which have some important practical advantages. For illustrative purposes in the present discussion, we will describe an approach which adjusts the controls one at a time, sometimes referred to as a grid search algorithm.

Assume that the laser is in an initial alignment state which is only required to be good enough so that the laser is functioning to allow the efficiency and bandwidth to be measured. Then assuming that we choose to optimize the BCD adjustment first:

1) compute the merit function value in the starting alignment

2) adjust the BCD by an amount D, where the size of D is chosen from experience to have a moderate effect on laser performance (i.e., a change big enough that it is not masked by measurement noise)

3) if the merit function improves after step 2, continue adjusting the BCD in increments of D until it gets worse

4) if the merit function gets worse after step 2, then continue adjusting the BCD in increments of −D until it gets worse.

5) At this point there will be 3 consecutive settings of the BCD, denoted D 1 , D 2 , and D 3 , with 3 values of the merit function M 1 , M 2 , and M 3 , where the middle value M 2 is better than the first M 1 or last M 3 . In general the best setting will not coincide with D 2 . We then use inverse parabolic interpolation to estimate the best setting using the formula: D optimum = D 2 - 1 2 * ( D 2 - D 1 ) 2  [ M 2 - M 3 ] - ( D 2 - D 3 ) 2  [ M 2 - M 1 ] ( D 2 - D 1 )  [ M 2 - M 3 ] - ( D 2 - D 3 )  [ M 2 - M 1 ]

and then set the BCD adjustment to D optimum . While it would be simpler to set the BCD to D 2 , computing the optimum in this way can significantly improve the results.

For the remaining adjustments, go to step 1, substituting each remaining adjustment (RMAX, CH-front, CH-rear) in turn where BCD is specified.

After all the adjustments have been optimized one at a time, then the adjustment increment D is reduced by some factor, for example, dividing by 2, for each of the adjustments, and this cycle is repeated. This is continued until the adjustment increment D becomes smaller than a predetermined level, and at this point the laser adjustment has been optimized.

Preferably, the optimization program will also be configured to keep beam parameters within specification. This can be done by providing an additional routine which checks the important quality parameters from time to time and if any go out of specification, the settings are adjusted to bring the out of specification parameter back within specification.

Measurement of Additional Beam Parameters

In order to measure various beam parameters, Applicants provided the optical setup described in FIG. 8 . An image of the laser beam at the output coupler aperture was relayed optically through lens 70 to a fluorescent screen and beam parameters including vertical and horizontal symmetry were determined utilizing a CCD camera focused on fluorescent screen 74 as shown in FIG. 8 . The fluorescent screen converts the UV light from the laser to visible light which is monitored by the CCD camera. The analog output from the camera is converted to digital with a video frame grabber and the output of the frame grabber is analyzed by a computer processor.

Applicants in conjunction with this work were also able to monitor beam divergence, beam pointing and beam pointing stability with a second beam path through lens 72 as shown in FIG. 8 . In this case, lens 72 focuses the laser beam onto the fluorescent screen 74 and is located so that perfectly colluminated light entering the lens would appear as a diffraction limited spot at the fluorescent screen. Therefore, the size of the spot is a measure of the divergence of the beam and movement of the spot is a measure of changes in beam pointing. These additional parameters could be used with the present invention to optimize laser performance taking into consideration these parameters.

PZT Deformable Grating

FIG. 9 shows a perspective view of the principal elements of a piezoelectric driven deformable grating assembly 80 for providing much greater control over the surface curvature of the grating as compared with the simple convex and simple concave curved surfaces produced by the FIG. 4 mechanism and also the potential for very rapid response times. The embodiment includes a grating 82 having dimensions of 12.7×35×250 mm and having a grating substrate comprised of ULE material. The grating surface is prepared using well known techniques for use in an eschelle configuration for the line narrowing of an excimer laser such as a KrF or ArF excimer laser. Seven invar rods 84 are attached with epoxy to the back of grating 82 . Each of the rods pass through, (1) a piezoelectric transducer 86 , (2) backing block 88 comprised of ULE material, (3) compression spring 90 and (4) nuts 92 all of which when assembled to pre-compress piezoelectric transducer 86 with a compression force of about 250 N. Backing block 84 is attached using epoxy on each and to invar baseplate 94 . Preferably the rods, transducers, backing block, spring, and nuts are assembled and the precompression is supplied to each piezoelectric transducer before the rods are epoxied to the back of the grating.

A recommended transducer 86 is Model P-305.00 available from Physik Instruments with offices in Germany with or Polytec Pi, Inc. with offices in Costa Mesa, Calif. The transducer has an ID of 5 mm and an OD of 10 mm and is 8 mm in length. It has a travel length of 5 μm with a 0-1000 V drive. Only a small fraction of this range is utilized so the 150 volt drive units (not shown) would normally be satisfactory. FIG. 9A shows a perspective view of the back of the grating assembly fully assembled and FIG. 9B shows a front perspective view and FIG. 9C shows a top view and FIG. 9D shows cross section at the 9 D location shown in FIG. 9 C.

Perspective sketches FIGS. 10A, 10 B, 10 C and 10 D show very exaggerated results of applying various forces at the seven rod stations. Table I indicates the individual forces and the resulting displacements at each rod station. Thus, as shown in the sketches, this embodiment can shape the surface into shapes of (a) a twisted surface, (b) a W-curved surface, (c) a S-curve surface or (d) a concave surface. Obviously, many other shapes could be produced with this embodiment. The reader will also understand that additional piezoelectric drives could be used to provide additional degrees of surface control or fewer could be used to reduce complications and costs.

The present invention is valuable to two main purposes. First, many gratings after fabrication are not usable for effective laser wavelength control and line narrowing because of lack of flatness of the grating surface. Sometimes, this lack of flatness approximates a concave or convex shape and can be corrected with the embodiments shown in FIG. 4 . But many times the grating has a twisted shape, W or S shape or other more complicated shape which the FIG. 4 embodiment could not fix. The present invention could returned many of these factory distorted shapes to a flat surface.

A second advantage of the FIG. 9 embodiment over the FIG. 4 embodiment results from the fact that the wavefront from the laser is often more complicated than merely flat, convex or concave so that good correction is not available with the FIG. 4 embodiment but would be available from the FIG. 9 embodiment.

A third advantage of the FIG. 9 embodiment over the FIG. 4 embodiment is that the curve correction in the FIG. 9 embodiment can be extremely fast. Adjustment can be made at intervals of about one millisecond or less.

Control of the PZT Deformable Grating

Various techniques can be used to control the surface of grating 82 to achieve desired results. In the lab a factory distorted grating surface can be monitored with an interferometer to determine its surface shape and the shape can be adjusted manually by trial and error to remove much of the distortion.

A better approach is to provide a feedback control of the positions of the driver rods to provide a grating surface needed for desired laser beam quality. A computer controlled feedback arrangement in which one or more beam parameters are monitored and data from this monitoring is used to control the positions of the seven driver rods. Many feedback control techniques are available for finding the best combination of piezoelectric rod position to produce the best beam quality. Some techniques of this type are described in the book by WH Press referred to earlier. A simple preferred technique would be to use the approach described above under the heading “An Example of an Optimization Technique”. In this case the four beam parameters: energy efficiency, wavelength stability, energy stability and bandwidth would be used to control the position of the driver rods. Initially all seven rods would be adjusted to seek the best combined values based on the assigned W values. After the initial adjustment for faster control, either one or perhaps three rods could be adjusted under a fast feedback technique. Then at longer intervals all seven rods could be optimized. Alternatively, optimization times could be chosen based on operating events, such as after a gas injection.

Multiple Grating Sections

The precise control of the grating surface position of the grating assembly shown in FIGS. 9 and 9A, B, C, and D permits the combination of grating sections as shown in FIG. 9E to produce an effectively longer grating using multiple individual gratings. Fabrication of high quality gratings longer than about 10 inches is very difficult and aligning two separate gratings side by side so that the grating lines match up is extremely difficult. However, with the extremely fine control of the surface position of the assembly shown in FIGS. 9 and 9A, B, C, and D, two separate gratings can be easily combined to provide a longer grating using multiple individual gratings. Since the laser beam is illuminating the surface at an angle, any position error in the lengthwise direction of the gratings can be compensated for by a forward or backward adjustment of the surface of one of the gratings so that the differences in distance traveled by the laser beam in the beam direction between each of the lined reflecting surfaces of the grating sections are all equal to multiples of the target wavelength value.

Thus the two separately fabricated gratings are assembled as shown in FIG. 9 then carefully assembled end-to-end as shown in FIG. 9 E. The PZT driver rods 84 then move one or both of the lined surfaces of the gratings until the lines of both gratings match up with each other so they are selecting the same narrow wavelength band.

PZT Deformable Mirror

Mirror 14 shown in FIG. 5 could be actively deformed using the techniques described above with reference to FIGS. 9 and 9 A-D. Preferred techniques use to control the deformation of the mirror could also be similar to the techniques described above using beam quality feedback information.

Other Deformation Techniques

Other grating or mirror deformation techniques could be utilized. One such additional technique is shown in FIG. 9 F. In this case piezo strain actuators 86 A are used to cause very slight bending of grating mirror 82 A. These strain type piezo wafers are available from Active Control experts, Inc. with offices in Cambridge, Mass. The wafers are applied in accordance with instructions from the piezo vendor.

Fast Tuning with Tuning Mirror

FIG. 11 A and FIG. 11B show the features of a technique for very fast control of the tuning mirror 14 position. In this embodiment, stepper motor 15 pivots mirror 14 through the action of lever arm 100 which pivots on carbide ball 101 which rests on fulcrum block 102 to push dowel pin 104 against four carbide balls 105 attached to mirror mount 14 A to produce very small movements of mirror 14 . Even finer faster adjustment of mirror 14 is provided by the action, indicated by double headed arrow 106 , of piezoelectric transducer 108 to cause very small changes in the position of fulcrum block 102 . Drum 110 isolates piezoelectric transducer 108 from the ultraviolet and ultra clean environment inside the line narrow wing module. Also shown in FIG. 11B is glue bond 112 PZT mount 114 securing screw 116 and electrical connection 118 .

Thus, the tuning equipment shown in FIGS. 11A and 11B give very fast mirror position control provided by the PZT drive) over a very limited radial range over the full required radial range is provided by the slower but greater range of stepper motor 15 and lever arm 100 . Control of the mirror position is provided with a feedback computer controller based on center line wavelength measurements made of the output beam by beam output monitor 22 as shown in FIG. 2 . Appropriate voltage may be applied to PZT drive 108 is to keep the wavelength values within a desired range and stepper motor lever arm may be positioned to keep the voltage driving PZT drive 108 within a desired range.

Although this invention has been disclosed and illustrated with reference to particular embodiments, the principals involved are susceptible for use in numerous other embodiments which will be apparent to persons skilled in the art. For example, each of the stepper motors could be replaced with alternative positioner units such as, ac or dc motors or hydraulic or pneumatic positioners. Many methods of controlling the positioners other than the suggested computer programs could be utilized. One or more stepper motors could be applied to the output coupler to automatically position it using similar techniques as described above for the R MAX mirror. Three strong permanent magnets could be used in the place of the two compression springs with one of the magnets replacing the piston as shown in FIG. 6 . Magnet 60 is fixed to rod 4 and magnets 62 and 64 are fixed to housing 8 . Rod 4 passes through holes in magnets 62 and 64 . The effect of screwing rod 4 into and out of housing 8 is substantially the same as described above. The curvature of the grating could be accomplished using any of a large number of techniques. For example, compression or tension could be applied at a number of points to create virtually any shape desired and these shapes could be subject to feedback computer control. Also, the tuning mirror 14 could be a deformable mirror subject to deformation to any desired shape in order to correct wave front distortions. The deformation could be manual or it could be piezoelectric or motor controlled and as with the grating it could be controlled in a feedback circuit. The invention is therefore to be limited only as indicated by the scope of the appended claims and their legal equivalents.